5 Extending The Game
The most obvious way to extend this game is to add a few more levels after the first 1. You can duplicate the first room to make a new level. Make sure that only the first level has the obj_game_controller in it, otherwise the lives and score will be reset every level.
The most obvious difference between levels is the number of asteroids. They usually increase by 1 per level.
Start & End Screens
Like all games, a small amount of 'packaging' makes a difference to the playing experience. More informative screens at the beginning and end of the game would make the experience better. High score tables are quite easy to implement in Game Maker - and would suit this type of game.
Some features of the classic game have not been implemented. A hyperspace button could make the ship disappear and reappear at a random location on the screen.
It is also normal to have a UFO appear every now and then, shooting at the ship until it leaves the screen. Big points are awarded for shooting the intruder.
Redoing The Graphics
There are lots of ways to do this. Perhaps you could make a set of retro graphics so that the game looks just like the classic version. How about having flames shoot out of the back of the ship when it is thrusting and not when it is still?